In ultrasonic medical images, the resolutions are divided into an axial resolution and a lateral resolution. Generally, the axial resolution relates to the spectral width of ultrasonic beams, and a signal in the form of a pulse having a small temporal width is used so as to increase the axial resolution. Generally, the lateral resolution is determined by the width of a main lobe and is dependent on the size of a transducer and the used frequency, and the beams spread as ultrasonic waves advance inside a medium due to diffraction. Accordingly the lateral resolution is lower than the axial resolution. As a method for increasing the lateral resolution of an ultrasonic image, a focusing method is used.
For the focusing, there are a method in which beams are collected at a fixed focusing point using an acoustic lens and a method in which beams are electrically focused using a multi-channel array transducer.
When a multi-channel array transducer is used, by applying mutually-different time delay values to transducers, the focusing point may be changed to an arbitrary position, and a received signal may be dynamically focused for all the points on scanning lines. This is called a receiving dynamic focusing method, and, at this time, the transmit-focusing point, generally, is fixed as one.
In order to acquire an effect of collecting transmitted beams in the whole area, a synthetic aperture beamforming method may be used in which several signals separately transmitted and received overlap to be synthesized.
In the synthetic aperture beamforming method, an image is synthesized after all the scanned data, which is transmitted and received from one element each time, acquired by using a linear array transducer is stored in a memory.
In the case of a focusing method in the B-mode, focusing at the transmission may be performed only at a depth of one point, and, as focusing at the reception, dynamic focusing is used in which focusing is performed at all image depths through signal processing. Accordingly, while high resolution is implemented at a position near the transmit-focusing point, at a depth deviated from the focal point, the transmitted sound field has a large beam width due to diffraction, and accordingly, the resolution is low.
In a case where the synthetic aperture beamforming method is applied to an ultrasonic imaging apparatus, the resolution is further higher than that of an image acquired using a conventional beamforming method as the number of beams to be synthesized increases.
However, in the case of the synthetic aperture beamforming method, a condition of an element pitch, which is a gap between the centers of elements, being λ/2 or less (here, λ=velocity of acoustic wave/center frequency) needs to be satisfied. When the condition is not satisfied, a grating lobe occurs, whereby the image quality deteriorates.
Generally, the synthetic aperture beamforming method is applied to a linear array probe or a convex array probe, and a gap between elements of a commercial linear array probe or a convex array probe is equal to or greater than λ.
In addition, even in the case of a probe that is produced to have an element gap of λ/2 for a synthetic aperture, when there is a motion, in order to reduce the artifact, a sparse array may be applied, and a gap between elements used at this time may be λ or more.
In a case where the sparse array is used, elements are used with elements corresponding to a predetermined constant being skipped when the whole image is acquired, and accordingly, a time required for acquiring one frame is shortened, whereby the motion artifact is reduced.
However, according to the synthetic aperture beamforming method, in a case where the condition of λ/2 is not satisfied, there is a disadvantage in that a grating lobe occurs at a low depth (0 to 4 cm), and thus, means solving such as problem is necessary.
In using the synthetic aperture beaming method, in order to improve the signal to noise ratio (SNR), a virtual transmission sound source technique may be used. In such a case, since weak energy arrives at an area between two virtual transmission points adjacent to each other so as to generate a shadow area, there is a disadvantage in that additional compensation therefor needs to be performed, and accordingly, means solving the problem is necessary.